The present invention relates to a semiconductor device which has a crystalline semiconductor material and a method for manufacturing the same.
Thin film transistors (TFTs) are known which utilize a thin film semiconductor formed on a substrate. The TFTs are utilized in integrated circuits, especially in an electro-optical device such as an active matrix type liquid crystal device as switching elements for each pixel or as driver elements in a peripheral circuit for driving active matrix elements.
Amorphous silicon films are readily available for TFTs. However, the electrical characteristics of amorphous silicon films are low. For this reason, it is desired to use semiconductor films having a crystallinity, that is, polycrystalline, microcrystalline silicon, monocrystalline semiconductor or the like.
As a method for forming silicon films having a crystallinity (crystalline silicon, hereinafter), it is known to deposit an amorphous silicon film first and then crystallize i applying heat or light energy such as laser light.
However, in the case of using heat energy, it is necessary to heat a substrate to a temperature 600xc2x0 C. or higher for more than 10 hours. For example, a Corning 7059 glass which is generally used as a substrate for an active matrix type liquid crystal device has a glass distortion point of 593xc2x0 C. Accordingly, the crystallization through such a high temperature heat treatment is not desirable for a class substrate. On the other hand, a short pulse laser such as an excimer laser has an advantage that it does not cause a distortion in a glass substrate. However, the uniformity of device characteristics is not so good in the case of using a laser. The inventors of the present invention considered that this is because of a temperature distribution in a laser beam.
The inventors of the present invention investigated a method for promoting a heat crystallization and a method for reducing a dispersion (ununiformity) in a laser crystallization in order to solve the problems concerning a crystallization of an amorphous silicon as discussed above.
With respect to the heat crystallization, it has been confirmed by the inventors that an amorphous silicon film can be crystallized through a heat treatment at 550xc2x0 C. for 4 hours by depositing a small amount of nickel, palladium, lead or the like on the silicon film.
As a method for introducing a small amount of the foregoing elements (i.e. a catalyst element for promoting crystallization), it is possible to use a plasma treatment, evaporation and ion implantation. The plasma treatment is a method in which a plasma of nitrogen or hydrogen is produced using an electrode including the catalyst element in a parallel plate type or positive columnar type plasma CVD apparatus, thereby, adding the catalyst element into an amorphous silicon film.
However, it is not desirable if the foregoing elements exist in a semiconductor too much because reliability or an electrical stability of a semiconductor device using such a semiconductor is hindered. Accordingly, the inventors have found that catalyst elements need to be used for crystallizing an amorphous silicon but it is desirable that a concentration of the catalyst elements in the crystallized silicon film be minimized. In order to achieve this object, it is desirable to use a catalyst element which is inactive in a crystalline silicon, and to accurately control the amount of the catalyst to be added into the silicon film in order to minimize the concentration of the catalyst element therein.
The crystallization process using a plasma treatment for adding nickel as a catalyst was studied in detail. The following findings were obtained as a result:
(1) In case of incorporating nickel by plasma treatment into an amorphous silicon film, nickel penetrates into the amorphous silicon film to a considerable depth before subjecting the film to a heat treatment;
(2) An initial nucleation occurs from the surface of the film in which nickel is added; and
(3) When a nickel layer is formed on the amorphous silicon film by vapor deposition, the crystallization of an amorphous silicon film occurs in the same manner as in the case of effecting the plasma treatment.
In view of the foregoing, it is assumed that not all of the nickel introduced by the plasma treatment functions to promote the crystallization of silicon. That is, if a large amount of nickel is introduced, there exists an excess amount of the nickel which does not function for promoting the crystallization. For this reason, it is the point or the face of the silicon which contacts nickel that functions to promote the crystallization of the silicon at lower temperatures. Further, it is concluded that nickel has to be minutely dispersed in the silicon in the form of atoms. Namely, it is assumed that nickel needs to be dispersed in the vicinity of a surface of an amorphous silicon film in the form of atoms, and the concentration of the nickel should be as small as possible but within a range which is sufficiently high to promote the lower temperature crystallization.
A trace amount of a catalyst element capable of promoting the crystallization of silicon can be incorporated in the vicinity of a surface of an amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous concerning the controllability of the film, and is therefore not suitable for precisely controlling the amount of the catalyst element to be incorporated in the amorphous silicon film.
Next, with respect to a dispersion in a characteristics occurring in a laser crystallization, the inventors of the present invention found through experiments that this is caused by mainly the two reason, i.e. (1) a nonuniformity in a crystallinity due to a temperature distribution on a laser irradiated surface, and (2) the creation of crystal nuclei being contingent. Specifically, a laser beam generally has an intensity distribution in accordance with a gaussian distribution. The temperature of an amorphous silicon film is also in conformity with this distribution. As a result, during a crystallization of amorphous silicon through melting or partial melting, the crystallization must start at a region which has a lower temperature or a higher temperature dispersion than other regions because a crystallization occurs when a region from a melting condition to a solid phase. However, in practice, a crystal nuclei do not necessarily exist in such a region and therefore, there is a possibility that a supercooling region is formed. If such a supercooling region contacts crystal nuclei, a crystallization occurs explosively. Also, it is assumed that a uniform crystallization is difficult because the crystal nuclei tend to be formed at a surface roughness of an interface with a silicon oxide.
Accordingly, it is desired that a region at which temperature firstly becomes below a melting point among other regions is in conformity with a region in which crystal nuclei exist.
It is an object of the present invention to obtain a crystalline semiconductor film with a high uniformity. More specifically, in view of the foregoing circumstances, it is an object of the present invention to control the formation of crystal nuclei in a silicon film.
In accordance with one aspect of the present invention, crystal nuclei are introduced into at a predetermined region of an amorphous silicon film following which a laser crystallization is carried out. When the crystal nuclei have a higher transmission rate with respect to the laser light than the amorphous silicon and have a higher thermal conductivity, the temperature of the film becomes lower than the melting point first at the crystal nuclei and therefore the crystallization starts there and a uniform crystalline film can be obtained. As the crystal nuclei, it is desirable to use a material which allows an epitaxial growth of silicon, for example, minute crystallites of silicon, or nickel silicide which is formed by adding nickel to an amorphous silicon film and then heating it.
Furthermore, it is desirable to add crystal nuclei not uniformly within a silicon film but on an upper or lower surface of the film. The addition of the crystal nuclei onto the surface of the silicon film is appropriate because the crystallization proceeds sufficiently in a thickness direction of the film. Also, this is considered helpful for enlarging a size of each crystal.
Moreover, it is desirable to irradiate a laser light from the side of the silicon film on which the crystal nuclei are added. By doing so, it is possible to remarkably reduce a surface roughness after the laser irradiation as compared with the case in which only a laser irradiation is used without the formation of the crystal nuclei. The inventors of the present invention consider this is because the absorption efficiency of the laser light by the crystal nuclei (i.e. crystal silicon) is smaller than that by an amorphous silicon so that this portion does not melts easily. Surface roughness is comparable with that in the case of using only a solid phase growth. Generally, the surface roughness is detrimental for a semiconductor device such as a TFT, for example, it causes a scattering of carriers.
In accordance with one embodiment of the present invention, a method for manufacturing a semiconductor device comprises the steps of:
forming an amorphous silicon film:
introducing crystal nuclei to said amorphous silicon film; and
growing crystals from said crystal nuclei, thereby obtaining a crystalline silicon film.
The crystal nuclei are formed by adding a catalyst including a catalyst element such as nickel onto a surface of the amorphous silicon film and then applying energy by heating or light irradiation (IR light irradiation). Further, the crystals grow from the introduced crystal nuclei by irradiating a laser light or a light equivalent to the laser light from the side on which the crystal nuclei are formed. The growth of the crystals is epitaxial.
As a method for adding a catalyst element, it is appropriate to coat an amorphous silicon film with a solution which contains the catalyst element therein. In particular, the catalyst element should be added by contacting the surface of the amorphous silicon film. This is important for accurately controlling the amount of the catalyst element to be incorporated into the films
The catalyst element may be added either from an upper surface or a lower surface of the amorphous silicon film. In the former case, the solution should be applied onto an upper surface of an amorphous silicon film after the deposition thereof. In the latter case, the solution should be applied onto a base surface and then the amorphous silicon film should be formed thereon.
The crystalline silicon film in accordance with the present invention is suitable as an active region of a semiconductor device which has at least one electrical junction such as PN, PI, NI or the like. For example, thin film transistors, diodes, photosensors may be manufactured.
The present invention has the following advantages:
(a) It is possible to accurately control and reduce the concentration of a catalyst element in the silicon film.
(b) If the solution contacts a surface of an amorphous silicon film, the amount of the catalyst element to be incorporated into the silicon film is determined by the concentration of the catalyst element in the solution.
(c) It is possible to introduce the catalyst element into the amorphous silicon film at a minimum density since the catalyst elements which are adsorbed by the surface of the amorphous silicon film function to promote the crystallization.
(d) A crystalline silicon film having a good crystallinity can be obtained without a high temperature process.
The catalyst provided by the solution may be in the form of a compound or in the form of atoms. Also, it may be dissolved in the solution, alternatively, it may be dispersed in the solution.
In the case of using a polar solvent such as water, alcohol, acid or ammonium, it is possible to use the compounds for adding nickel, namely, nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetly acetonate, 4-cyclohexyl nickel butyric acid, nickel oxide and nickel hydroxide.
Also, benzene, toluene, xylene, carbon tetrachloride, chloroform or ether can be used as a non-polar solvent. Examples of nickel compounds suitable for a non-polar solvent are nickel acetyl acetonate and 2-ethyl hexanoic acid nickel.
Further, it is possible to add an interfacial active agent to a solution containing a catalytic element. By doing so, the solution can be adhered to and adsorbed by a surface at a higher efficiency. The interfacial active agent may be coated on the surface to be coated in advance of coating the solution.
Also, when using an elemental nickel (metal), it is necessary to use an acid to dissolve it.
In the foregoing examples, the nickel can be completely solved by the solvent. However, even if the nickel is not completely solved, it is possible to use a material such as an emulsion in which elemental nickel or nickel compound is dispersed uniformly in a dispersion medium. It is also possible to use a solution which is for forming a silicon oxide film. An example of such a solution is OCD (Ohka Diffusion Source) produced by Tokyo Ohka Kogyo Kabushiki Kaisha. A silicon oxide film may be easily formed by coating the OCD on a sur ace and then baking at about 200xc2x0 C. It is also possible to add desired impurities to the silicon oxide film.
When using a polar solvent such as water for dissolving nickel, it is likely that an amorphous silicon film repels it. In such a case, a thin oxide film is preferably formed on the amorphous silicon film so that the solution can be provided thereon uniformly. The thickness of the oxide film is preferably 100 xc3x85 or less. Also, it is possible to add an interfacial active agent to the solution in order to increase a wetting property.
When using a non-polar solvent such as toluene for obtaining a solution of 2-ethyl hexanoic acid nickel, the solution can be directly formed on the surface of an amorphous silicon film. However, it is possible to interpose a material between the amorphous silicon film and the solution for increasing the adhesivity therebetween, for example, OAP (containing hexamethyl disilazane as a main component, produced by Tokyo Oka Kogyo) which is used to increase adhesivity of a resist.
The concentration of the catalyst element in the solution depends on the kind of the solution, however, roughly speaking, the concentration of the catalyst element such as nickel by weight in the solution should be 0.01-10 ppm, preferably, 0.01-1 ppm. The concentration is measured based on the nickel concentration in the silicon film after the completion of the crystallization.
After forming crystal nuclei by carrying out a heat treating on an amorphous silicon film which is added with a catalyst element, the silicon film can be uniformly crystallized into a crystalline silicon film by the use of a laser irradiation.
When a laser crystallization is carried out on an amorphous silicon film without crystal nuclei, the power of the laser necessary for the crystallization is much higher than the laser crystallization in which crystal nuclei are previously formed. Conventionally, it was known that a laser power necessary for crystallizing an amorphous silicon film having microcrystallites is higher than the laser power necessary for crystallizing an amorphous silicon film having no crystallinity (because the difference in absorption efficiency of a laser light by the silicon films). However, the present invention is entirely opposite to this since a lower laser power is sufficient for crystallizing a silicon film in which crystal nuclei are formed.
In the present invention, the region of the silicon film which become a crystal nucleus upon crystallization can be controlled by controlling the amount of the catalyst element incorporated into the film. The film can be regarded as in a state which is a mixture of a crystalline structure and an amorphous structure. Typically, a proportion of crystal components with respect to the entire plane of the film is from 0.01 to 20%. By the application of a laser light in this state, crystals can grow from the crystal nuclei which exist in the regions having crystallinity, and accordingly, it is possible to obtain a hither crystallinity. In other words, small crystallites are grown into large crystallites. For this reason, the crystal growth length, the size and number of crystallites, or the like can be controlled by controlling the amount of a catalyst element and the power of a laser light.
Instead of using a laser light, it is also possible to use an intense light, especially, an infrared light for crystallization. Since infrared ray is not so absorbed by a glass substrate, it is possible to heat only the silicon film. This irradiation is generally called as a rapid thermal annealing (RTA) or rapid thermal process (RTP).
In the present invention, nickel is disclosed as a most preferred catalyst element. However, it is to be understood that other catalyst elements may be used in a similar manner. Examples of such elements are Pd, Pt, Cu, Ag, Au, In, Sn, Pb, P, As and Sb. It is also possible to select one or more elements from the groups VIII, IIIb, IVb and Vb elements of the periodic table.
In place of using a solution such as water or alcohol, it is also possible to use other materials which contain a catalyst material, for example, metal compound or oxide.